In these questions, the letters are linked in some way. Identify the relation and complete the last box. \[\text{B}\,\,\,\text{:}\,\,\,\,\text{2}\] \[\text{D}\,\,\,\text{:}\,\,\,\,\text{4}\] \[\text{M}\,\,\,\text{:}\,\,\,\,\text{?}\]

• A.  10         
• B.         5      
• C.  8            
• D.         13